This invention relates generally to microprocessor based controls for a demand defrost heat pump control. More specifically, the present invention relates to a method and apparatus for controlling a microprocessor controlled relay for a heat pump system which has the ability to determine when the microprocessor is reset due to relay switching noise or the microprocessor has just been powered-up and the program should start its initialization routine.
Heat pumps include a refrigeration circuit with a compressor and outdoor and indoor heat exchanger coils controlled to function alternately as an evaporator and a condenser in response to a thermostat controlled reversing valve calling for refrigerant to flow in one direction during heating cycles and in the opposite direction during cooling cycles. During the cooling mode the indoor coil functions as an evaporator absorbing surplus heat from the indoor air, and the outdoor coil functions as a condenser rejecting the heat into the outdoor air. During the heating mode the outdoor heat exchanger provides means to vaporize the liquid refrigerant by heat transfer from air flowing through the heat exchanger. Efficient operation of the system requires that sufficient heat be transferred from the air flowing through the outdoor heat exchanger to maintain adequate capacity to meet the heating demand of the conditioned space. When the outdoor temperatures are around 40.degree. F., colder moisture from the outdoor air is collected on the outdoor coil fins in the form of frost. The frost blocks air flow through the heat exchanger thereby reducing the heat exchanger's capacity for heat transfer below that required to meet the heating demand in the conditioned space. It is therefore common practice to defrost the outdoor heat exchanger, melting the accumulated frost to prevent an unacceptable level of heat transfer degradation.
The frost accumulation is removed by temporarily operating the heat pump in a cooling cycle, wherein hot refrigerant vapor from the indoor coil is pumped through the compressor to the outdoor coil to heat it for frost removal. The defrost cycle is periodically implemented by numerous techniques known in the art. Generally, all defrost controls incorporate relays to switch off the outdoor fan when entering a defrost cycle, to switch on the reversing valve solenoid when entering the defrost cycle, and to switch on strip heater sequencers when entering the defrost cycle. However, in a microprocessor control, transient electro-magnetic noise can cause an unwanted failure in the control. This problem is especially troublesome when the control, by switching relays with inductive coils, causes the microprocessor to reset, i.e. the processor program stops and restarts at its beginning, generally causing a discontinuous control operation.